STP and VLAN tagging

Discussion in 'Cisco' started by uofitorn@gmail.com, Mar 29, 2005.

Guest

Hello everyone,
I have two switches that are tagged and trunked together with two
connections. I want to manually configure the STP priorities on the
VLANs that are on these switches to ensure a nice deterministic
convergence. I understand that each VLAN runs its own instance of STP
and is seperately configured at the VLAN level. But how do I configure
the priority for the VLAN that is trunked between the switches? Do I
configure the same priority in each switches' configuration for the
VLAN that is on both switches?

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The switch with the lowest priority will be the root switch. Once
you've figured out which switch you want to be the root then use the
"set spantree priority" command on it to make it less than the default
of 32768 (assuming the other switches haven't been changed). It's a
good idea also to have a backup root switch.
Cisco uses Per-VLAN spanning tree (PVST). The initial version of .1q
specified using a single instance of STP for all VLANs but they may
have added support for multiple instances. Maybe someone else knows
this.

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<> wrote in message
news:...
> Hello everyone,
> I have two switches that are tagged and trunked together with two
> connections. I want to manually configure the STP priorities on the
> VLANs that are on these switches to ensure a nice deterministic
> convergence. I understand that each VLAN runs its own instance of STP
> and is seperately configured at the VLAN level. But how do I configure
> the priority for the VLAN that is trunked between the switches? Do I
> configure the same priority in each switches' configuration for the
> VLAN that is on both switches?
>
> Thanks!
>

It depends on how you want it to work. For my purposes, I want a single
switch to be the root bridge. Therefore, I set the priority on that switch
and that is all that needs to be done. For example:

spanning-tree vlan 10,20,30,40,50,60 priority 4096

If you don't set this on a switch, it will use the default prio+mac address
for the bridge and will calculate automatically. Setting the same priority
for a vlan between the switches would be the same as not setting the
priority at all. If you have certain areas that you want to have separate
root bridges, set the priority for the particular vlans on that device.

That said... With only two switches, you don't really need spanning-tree
unless you are using two uplinks for redundancy. Even so, I'm not sure I
would even care about the root bridge as it really wouldn't make any
difference.

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